The pressure of water varies with the depth of the water, but the pressure of air is substantially not dependent on depth. This fact gives rise to a limitation on this "cascading-canister" system (more generally known now as the CASCAN (TM) system) and that is that, for the structure of each canister to be relatively unstressed, the individual canisters should be quite short in height. It is only at the level where the water actually contacts the air that the air and water pressures are exactly equal. If the canister were for example 10 meters high, then there would be a pressure difference between the air inside the top of the canister and the water outside the top of the canister of around one atmosphere, and the walls of the canister would have to be strong enough not to explode under that pressure.
Another reason why the canister should be short is that the canister must be airtight. The bigger the canister, the more difficult a production problem there may be to ensure the integrity of the structure.
Each section of the strut should have its own weight supported by the canister or canisters of air associated with that section. In other words, the sections of the strut should each be neutrally buoyant. Steel has a density of about seven and a half times that of the (salt) water that is to be displaced by the air. In the case therefore where the canister is only, say, half the height of the section, the cross-sectional area of the air space in the canister will have to be fifteen times as large as the cross-sectional area of the steel.
A canister as wide as that is too bulky to be economically manufactured. If one uses more than one canister, the problems on the ship, during deployment of the strut, of mounting the canisters to the sections are too much. The problem arises because it is not economically permissible to make attachments to the steel of the strut section at any point in the section other than right at the ends. It is acceptable to make attachments at the ends since the ends have to be formed with bulbous flanges in any case because of the joints. The main part of the length of the section is slender, and highly stressed. Its surface has an anti-corrosion coating that is to be carefully examined for scratches and cracks and other imperfections or damage to the coating that could be stress-concentration points or give rise to other problems. It is only at the bulbous ends that these precautions can be relaxed and, for example, holes made in the steel. The canisters should not even be allowed to chafe against the surface coating, and clamp-on collars are not permissible either.
One could conceive of using a flange at the bulbous junction, and allowing a canister to float up underneath and against the flange, and then allowing another canister to float up underneath and against the first canister. This too is unacceptable, because the buoyancy upthrust of the lower canister could crush the upper canister.
Virtually all these problems of manufacture and of ensuring a long reliable life of the canisters and the sections might be overcome if it were economical to make the components thicker, stronger, and larger. However, there is yet a further very difficult problem, and that is the problem of the speed of deployment of the strut. A strut is deployed section by section from a ship, the strut gradually becoming longer until it touches the bottom. Good weather is needed throughout assembly, as it is not economically permissible to break off before deployment of the strut is finished. The predictable weather window is small, and assembly and installation must be finished within it. The speed at which the sections can be hoisted into position, joined, and the canisters added, is therefore critical.
It has been found not to be economically possible using conventional methods to produce a tensile strut by which a platform may be tethered to the sea bed, because of the problems outlined above. Tension leg platforms (TLP's) however, are thought by many to be the best basis for the future exploitation of undersea resources in very deep water, if only the tensile struts, or tethers, could be economically made and deployed.